Thermosetting resins are widely used in the formation of fiber-reinforced structural materials due to their ability to withstand high temperatures. These materials provide the high strength and high temperature capability needed in such applications as military aircraft.
Thermosetting resins which have been used in this context include epoxy resins and bismaleimide resins. The latter are particularly beneficial due to their high glass transition temperature their high capability of withstanding hot wet environments, and their low smoke and toxicant emission.
The beneficial properties of thermosetting resins are unfortunately offset by a brittleness or low toughness resulting in low damage tolerance. Toughness is extremely important in high-performance structures, since high loads must be borne both within the plane of the composite and in directions normal to the plane of the composite. Among the solutions offered for increasing toughness are the use of thermoplastics in combination with the thermosetting resins. This usually occurs, however, at the expense of processability, the thermoplastic additive causing an increase in the viscosity and the softening point of the material.